Creates a libdumpstateheaders module to export headers.

Currently, the device-specific dumpstate projects uses dumpstate.h
by importing just the header in the their makefiles:

LOCAL_C_INCLUDES := frameworks/native/cmds/dumpstate

But this approach makes it hard to include external stuff on
dumpstate.h (like common macros) because it breaks the device-specific
builds (unless they are changed to include the proper shared library
dependency).

A better approach is to define a libdumpstateheaders that contains the
proper dependencies, and let the projects use it instead:

LOCAL_STATIC_LIBRARIES := libdumpstateheaders

BUG: 26379932
BUG: 31982882
Test: DumpstateTest passes and manual verification

Change-Id: Iaabff379aded412f33cda1f93f3253866aff9f25